Any structure may suffer damage during its use that may lead to the eventual failure of the structure. In many scenarios, it is important to monitor damage so that the damage can be repaired or the structure can be replaced before any degradation of performance occurs. Many such structures are built and used in the aeronautical, aerospace, maritime, or automotive industries.
When damage occurs within a structure, the damaged area emits an acoustic emission (AE) that propagates through the material of the structure. Acoustic damage monitoring systems, in the form of acoustic emission detection and monitoring systems, are arranged to detect the acoustic emission made as damage occurs to a structure. Such systems are used in Non Destructive Testing (NDT) systems such as Structural Health Monitoring (SHM) systems. In such systems, sensors attached at known locations in the structure detect the acoustic emissions. The time of flight (ToF) of the acoustic emission to each sensor is recorded. The location of the AE can then be determined using triangulation of the ToFs for a given AE from the known locations for the receiving sensors. Such techniques of detecting AEs are referred to as passive acoustic monitoring systems. Another type of acoustic monitoring system is referred to as an active system. In such active systems, a transducer attached to a given structure generates an interrogating acoustic signal and any received echo analysed to identify and quantify defects or damage.
In mechanical structures, such as aircraft sections or components, which are predominantly constructed of plates, the acoustic waves form particular types of plate waves known as Lamb waves. In passive systems the acoustic waves are emitted by damage as it occurs while in active systems the acoustic waves are emitted or generated by a transducer. Lamb waves have a number of different oscillatory patterns or modes that are capable of maintaining their shape and propagating in a stable or unstable manner depending on their dispersivity state. Changes in the mechanical form of a structure, such as a boundary between one material and another or changes in cross sectional thickness of a given material, can affect the Lamb wave signal. For example, a material joint may delay a Lamb wave signal, reduce its amplitude or change its mode. Different wave modes may be affected differently by such structural variations. For example, one Lamb wave mode may be attenuated differently to another mode by a given structural variation along the wave path. Indeed the attenuation of some modes may be so great that the given mode fails to reach a given sensor location with a detectable amplitude. Lamb waves propagate in all directions but are sensitive to the directional stiffness and thickness of the structure in which they travel. Thus, a given structure may facilitate propagation of Lamb waves in a particular direction. The stiffness and thickness may result from features within the structure.
Each Lamb wave mode commonly has a signature frequency and wavelength band. All modes may not reach the point at which a sensor for a passive or active monitoring system is located. Thus one problem is matching the frequency of a Lamb wave generating or sensing transducers located at a given point to the frequency bands likely to be detected at that point.